Currently, audio connectors may have three-contact or four-contact. Two different forms may found for the four-contact: International Standard (also see in OMTP, Open Mobile Terminal Platform) four-contact audio connectors and American Standard (also see in CTIA, Cellular Telecommunications Industry Association) four-contact audio connectors. For the International Standard four-contact audio connector, pin (also called as contact) 1 is for a left channel, pin 2 is for a right channel , pin 3 is for a MIC (microphone), and pin 4 is for ground return. While for the American Standard four-contact audio connector, the pins are arranged in a different way: pin 1 is for a left channel (also called as a left channel contact), pin 2 is for a right channel (also called as a right channel contact), pin 3 is for ground return (GND) (also called as a GND contact), and pin 4 is for a microphone (MIC) (also called as a MIC contact). Take an International Standard four-contact earphone plug as an example, a first terminal of a left channel is connected to pin 1, a second terminal of the left channel is connected to pin 4; a first terminal of a right channel is connected to pin 2, a second terminal of the right channel is connected to pin 4; a first terminal of a microphone is connected to pin 3, a second terminal of the microphone is connected to pin 4. Generally, an audio apparatus uses the four-contact audio connector and transmits audio signals to a host connected through a MIC pin, in which, in the audio apparatus, the communication of transmitting the audio signals to the host connected through the MIC pin is called as an audio communication channel
FIG. 1 is a circuit structure diagram illustrating an audio communication channel circuit of an audio apparatus in the related art. As shown in FIG. 1, in the related art, in order to make the audio apparatus be compatible with both International Standard audio connector and American Standard audio connector, two channels corresponding to pin 3 and pin 4 respectively in the audio apparatus are totally the same, that is, they both equal to providing an equivalent resistor (i.e. R1 and R2 in FIG. 1). The equivalent resistor is connected to pin 3 or pin 4 through a capacitor, that is, an output terminal of the audio communication channel in the audio apparatus is connected to pin 3 or pin 4 through the capacitor (i.e. C1 and C2 in FIG. 1). When the audio apparatus is inserted into the host, a switch S2 is configured to connect a pin (assuming pin 3) corresponding to a GND contact of the host to ground under effect of an electrical level input from a GFIO (General-Function Input/output Contact). An uploading input audio signal (AUpload1) from the audio apparatus to the host may pass through the output terminals of the two channels (EP_PIN3 and EP_PIN4) and capacitors C1, C2 respectively and reach to pin 3 and pin 4 of an audio interface. The audio signal is output to the host through a pin (shown as pin 4 in FIG. 1) which is connected to a MIC contact of the host. EP_PIN3 and EP_PIN4 are connected to an electronic element (shown as equivalent resistors R1 and R2 in FIG. 1) with high resistance (generally 27KΩ) in series respectively. When an interface (earphone plug) of the audio apparatus is inserted into the host (e.g. a computer or a mobile terminal), the host may identify an identification circuit (Rmic) which is connected EP_PIN3 with EP_PIN4 and achieve a detection of whether the audio connector of the audio apparatus is matching.
In the related art, the audio apparatus usually uses Frequency-shift keying (FSK for short) data. When the audio apparatus communicates with the host, a voltage output by the audio apparatus may vary after connected to the host. The variation amplitude of the voltage output by the audio apparatus may affect the quality of the signal from the audio apparatus to the host. The greater the variation amplitude of the voltage output is, the worse the quality of the signal comes. In the circuit structure shown in FIG. 1, an output terminal of the audio communication channel is connected to pin 4 via a capacitor (C2). When the audio apparatus is inserted into the host, an identification circuit and the host are connected in parallel and then connected to R2 in series on an audio communication channel which is connected to a MIC of the host. Since the resistance value of R2 and the resistance value of the identification circuit are both very high, when the audio apparatus is inserted into the host, the amplitude of the signal from the audio apparatus to the host is determined by the value of the equivalent resistance that the host connected to the audio apparatus. The host with a higher equivalent resistance may correspond to a signal with bigger amplitude; the host with a lower equivalent resistance may correspond to a signal with smaller amplitude. In practical application, there are all kinds of hosts. Some hosts have the low equivalent resistance value when connected to the audio apparatus, while some hosts have the high equivalent resistance value when connected to the audio apparatus. Therefore, the amplitude of the signal from the audio apparatus to the host is uncontrollable. Besides, for those hosts with a too high or too low equivalent resistance value, the amplitude of the signal from the audio apparatus to the host may be too great or too little so that go beyond the identification scope of the host, thus the scope of the host that the audio apparatus may be compatible with is limited.